ProSe Function ID

Description

The ProSe Function ID (PFID) is a globally unique identifier used within the 3GPP Proximity Services (ProSe) architecture to identify a specific ProSe Function instance. ProSe, introduced in Release 12, enables UEs that are in proximity to each other to discover one another and communicate directly, either with or without network assistance. The ProSe Function is the logical entity in the Evolved Packet Core (EPC) that provides all the necessary network-related actions for ProSe, including provisioning, authorization, and configuration of UEs for direct discovery and direct communication. Because a network may deploy multiple ProSe Function instances (for load distribution, geographic coverage, or administrative separation), a stable identifier is required so that UEs, eNodeBs, and other core network nodes can unambiguously reference the correct instance.

The PFID is used in several key procedures. During UE registration and authorization for ProSe, the UE is provisioned with the PFID of the ProSe Function that serves it, along with associated parameters such as the ProSe Function's address. When a UE performs ProSe direct discovery, the discovery messages and any network-assisted discovery requests are routed to the ProSe Function identified by the PFID. In the PC3 interface between the UE and the ProSe Function, and in the PC4a/PC4b interfaces between the ProSe Function and other EPC entities (such as the HSS and the MME), the PFID helps ensure that signaling is directed to the correct functional entity. The PFID is also used in inter-ProSe-Function communication, where multiple ProSe Functions may need to coordinate, for example when UEs served by different ProSe Functions engage in discovery or communication.

Architecturally, the PFID is defined in 3GPP TS 23.303 (Proximity-based services; Stage 2), which specifies the overall ProSe architecture and procedures. The PFID is a component of the ProSe Function identity and is typically formatted as a fully qualified domain name (FQDN) or a similar structured identifier, allowing it to be resolved to an IP address for signaling transport. TS 29.343 specifies the ProSe Function to ProSe Function (PC6) interface and the associated signaling, where the PFID is used to identify the source and target ProSe Functions. TS 33.303 covers the security aspects of ProSe, including how the PFID is protected and authenticated during discovery and communication procedures.

The PFID plays a critical role in ensuring that ProSe operations are correctly authorized and routed. Without a unique identifier, a UE that moves between areas served by different ProSe Functions could not determine which function to contact for discovery or communication authorization. Similarly, the network could not correlate ProSe requests with the correct subscriber context. The PFID therefore underpins the scalability and multi-vendor interoperability of ProSe deployments, allowing operators to deploy multiple ProSe Function instances while maintaining consistent addressing and security.

In later releases, the concept of the ProSe Function and its identifier has been extended to support additional features such as UE-to-Network Relay, UE-to-UE Relay, and ProSe for public safety (including mission-critical services). The PFID remains a foundational identifier in these enhancements, ensuring backward compatibility and consistent addressing across releases from Rel-12 through Rel-19.

Purpose & Motivation

The PFID exists to provide a unique, routable identifier for ProSe Function instances in the 3GPP Proximity Services architecture. Before ProSe was introduced in Release 12, there was no standardized way for UEs to discover and communicate directly with each other under network control. When ProSe was defined, the network needed a logical entity—the ProSe Function—to handle authorization, provisioning, and discovery coordination. Since an operator may deploy multiple such entities, a stable identifier was required so that UEs and network nodes could address the correct instance.

The PFID solves several problems. First, it enables unambiguous addressing in the PC3, PC4, and PC6 interfaces, ensuring that signaling reaches the intended ProSe Function. Second, it supports mobility: as a UE moves, it can be redirected to a different ProSe Function, and the PFID allows the network to update the UE's serving function. Third, it facilitates inter-operator and inter-function coordination, which is essential for public safety and commercial ProSe use cases where UEs may be served by different ProSe Functions or even different operators.

The motivation for the PFID is rooted in the need for scalable, secure, and interoperable ProSe deployments. Without it, multi-vendor and multi-instance deployments would be impractical, and procedures such as network-assisted discovery and direct communication authorization could not be reliably executed. The PFID thus provides the addressing foundation that makes ProSe a viable technology for both commercial and mission-critical applications.